docs: def cigtab(self, y):
          """Cigtab test objective function"""
          X = [y] if isscalar(y[0]) else y
          f = [1e-4 * x[0]**2 + 1e4 * x[1]**2 + sum(x[2:]**2) for x in X]
          return f if len(f) > 1 else f[0]